Wireless devices such as cellular telephones and Wi-Fi networking devices require the capability to transmit and receive wireless signals simultaneously. This is accomplished with a full duplex transceiver. FIG. 1 illustrates a conventional full duplex transceiver 100, which includes a signal input portion 102, a transmitter 104, an amplifier 106, an antenna 108, a duplexer 110, an amplifier 112 and a receiver 114.
The data to be transmitted 116 are passed from signal input 102 to transmitter 104. Output 118 from transmitter 104 is amplified by amplifier 106. Output 120 from amplifier 106 is supplied to duplexer 110, which generates transmit signal 121 for transmission out of antenna 108.
The transmission frequency of the transceiver is typically lower than the receiving frequency. Ideally, there is no overlap in the transmission frequency band and the receiving frequency band. In the event that antenna 108 receives a reception signal 122 while transmitting, duplexer 110 separates reception signal 122 from transmit signal 121 to generate receiver signal 124. Receiver signal 124 is amplified by amplifier 112. Amplified received signal 126 is then processed by receiver 114.
Any such full duplex transceiver has an inherent problem with self-interference: transmission interferes with reception in a process called transmission leakage even though there should be no overlap in the bands. Leakage results not only from imperfect duplexer performance in isolating the transmit signal from the reception signal, but also from parasitic coupling paths between multiple elements of the transmission circuitry and the reception circuitry which permit leakage of the transmission signal into the reception signal. Contemporary market pressures push for smaller and smaller transceiver circuits, thus compounding the problem as transmission circuitry is pressed ever closer to reception circuitry.
One solution to the generic problem of transmission leakage, called active cancellation, involves a feedback loop constructed around the low-noise amplifier that amplifies the reception signal. This feedback loop generates a cancellation signal based on input from the low-noise amplifier and from the transmission circuitry. The active cancellation system produces the cancellation signal such that it mimics the inverse of the transmission leakage; hence, by adding the cancellation signal to the reception signal the leakage present within the reception signal is ‘cancelled’. Commonly owned, U.S. patent application having Ser. No. 11/712,737, to Khurram Muhammad filed on Mar. 1, 2007, and titled “METHODS AND APPARATUS TO PERFORM NOISE CANCELLATION IN RADIOS,” discloses one type of active cancellation. The entire disclosure of Ser. No. 11/712,737 is incorporated herein.